<!DOCTYPE html>
<html>
<head>
    <title>JavaScript Loan Calculator</title>
    <style> /* This is a CSS style sheet: it adds style to the program output */
    .output {
        font-weight: bold;
    }

    /* Calculated values in bold */
    #payment {
        text-decoration: underline;
    }

    /* For element with id="payment" */
    #graph {
        border: solid black 1px;
    }

    /* Chart has a simple border */
    th, td {
        vertical-align: top;
    }

    /* Don't center table cells */
    </style>
</head>
<body>
<!--
  This is an HTML table with <input> elements that allow the user to enter data
  and <span> elements in which the program can display its results.
  These elements have ids like "interest" and "years". These ids are used
  in the JavaScript code that follows the table. Note that some of the input
  elements define "onchange" or "onclick" event handlers. These specify strings
  of JavaScript code to be executed when the user enters data or clicks.
-->
<table>
    <tr>
        <th>Enter Loan Data:</th>
        <td></td>
        <th>Loan Balance, Cumulative Equity, and Interest Payments</th>
    </tr>
    <tr>
        <td>Amount of the loan ($):</td>
        <td><input id="amount" onchange="calculate();"></td>
        <td rowspan=8>
            <canvas id="graph" width="400" height="250"></canvas>
        </td>
    </tr>
    <tr>
        <td>Annual interest (%):</td>
        <td><input id="apr" onchange="calculate();"></td>
    </tr>
    <tr>
        <td>Repayment period (years):</td>
        <td><input id="years" onchange="calculate();"></td>
    <tr>
        <td>Zipcode (to find lenders):</td>
        <td><input id="zipcode" onchange="calculate();"></td>
    <tr>
        <th>Approximate Payments:</th>
        <td>
            <button onclick="calculate();">Calculate</button>
        </td>
    </tr>
    <tr>
        <td>Monthly payment:</td>
        <td>$<span class="output" id="payment"></span></td>
    </tr>
    <tr>
        <td>Total payment:</td>
        <td>$<span class="output" id="total"></span></td>
    </tr>
    <tr>
        <td>Total interest:</td>
        <td>$<span class="output" id="totalinterest"></span></td>
    </tr>
    <tr>
        <th>Sponsors:</th>
        <td colspan=2>
            Apply for your loan with one of these fine lenders:
            <div id="lenders"></div>
        </td>
    </tr>
</table>

<!-- The rest of this example is JavaScript code in the <script> tag below -->
<!-- Normally, this script would go in the document <head> above but it -->
<!-- is easier to understand here, after you've seen its HTML context. -->
<script>
    "use strict"; // Use ECMAScript 5 strict mode in browsers that support it

    /*
     * This script defines the calculate() function called by the event handlers
     * in HTML above. The function reads values from <input> elements, calculates
     * loan payment information, displays the results in <span> elements. It also
     * saves the user's data, displays links to lenders, and draws a chart.
     */
    function calculate() {
        // Look up the input and output elements in the document
        var amount = document.getElementById("amount");
        var apr = document.getElementById("apr");
        var years = document.getElementById("years");
        var zipcode = document.getElementById("zipcode");
        var payment = document.getElementById("payment");
        var total = document.getElementById("total");
        var totalinterest = document.getElementById("totalinterest");

        // Get the user's input from the input elements. Assume it is all valid.
        // Convert interest from a percentage to a decimal, and convert from
        // an annual rate to a monthly rate. Convert payment period in years
        // to the number of monthly payments.
        var principal = parseFloat(amount.value);
        var interest = parseFloat(apr.value) / 100 / 12;
        var payments = parseFloat(years.value) * 12;

        // Now compute the monthly payment figure.
        var x = Math.pow(1 + interest, payments);   // Math.pow() computes powers
        var monthly = (principal * x * interest) / (x - 1);

        // If the result is a finite number, the user's input was good and
        // we have meaningful results to display
        if (isFinite(monthly)) {
            // Fill in the output fields, rounding to 2 decimal places
            payment.innerHTML = monthly.toFixed(2);
            total.innerHTML = (monthly * payments).toFixed(2);
            totalinterest.innerHTML = ((monthly * payments) - principal).toFixed(2);

            // Save the user's input so we can restore it the next time they visit
            save(amount.value, apr.value, years.value, zipcode.value);

            // Advertise: find and display local lenders, but ignore network errors
            try {      // Catch any errors that occur within these curly braces
                getLenders(amount.value, apr.value, years.value, zipcode.value);
            }
            catch (e) { /* And ignore those errors */
            }

            // Finally, chart loan balance, and interest and equity payments
            chart(principal, interest, monthly, payments);
        }
        else {
            // Result was Not-a-Number or infinite, which means the input was
            // incomplete or invalid. Clear any previously displayed output.
            payment.innerHTML = "";        // Erase the content of these elements
            total.innerHTML = ""
            totalinterest.innerHTML = "";
            chart();                       // With no arguments, clears the chart
        }
    }

    // Save the user's input as properties of the localStorage object. Those
    // properties will still be there when the user visits in the future
    // This storage feature will not work in some browsers (Firefox, e.g.) if you
    // run the example from a local file:// URL.  It does work over HTTP, however.
    function save(amount, apr, years, zipcode) {
        if (window.localStorage) {  // Only do this if the browser supports it
            localStorage.loan_amount = amount;
            localStorage.loan_apr = apr;
            localStorage.loan_years = years;
            localStorage.loan_zipcode = zipcode;
        }
    }

    // Automatically attempt to restore input fields when the document first loads.
    window.onload = function () {
        // If the browser supports localStorage and we have some stored data
        if (window.localStorage && localStorage.loan_amount) {
            document.getElementById("amount").value = localStorage.loan_amount;
            document.getElementById("apr").value = localStorage.loan_apr;
            document.getElementById("years").value = localStorage.loan_years;
            document.getElementById("zipcode").value = localStorage.loan_zipcode;
        }
    };

    // Pass the user's input to a server-side script which can (in theory) return
    // a list of links to local lenders interested in making loans.  This example
    // does not actually include a working implementation of such a lender-finding
    // service. But if the service existed, this function would work with it.
    function getLenders(amount, apr, years, zipcode) {
        // If the browser does not support the XMLHttpRequest object, do nothing
        if (!window.XMLHttpRequest) return;

        // Find the element to display the list of lenders in
        var ad = document.getElementById("lenders");
        if (!ad) return;                            // Quit if no spot for output

        // Encode the user's input as query parameters in a URL
        var url = "getLenders.php" +                // Service url plus
                "?amt=" + encodeURIComponent(amount) +  // user data in query string
                "&apr=" + encodeURIComponent(apr) +
                "&yrs=" + encodeURIComponent(years) +
                "&zip=" + encodeURIComponent(zipcode);

        // Fetch the contents of that URL using the XMLHttpRequest object
        var req = new XMLHttpRequest();        // Begin a new request
        req.open("GET", url);                  // An HTTP GET request for the url
        req.send(null);                        // Send the request with no body

        // Before returning, register an event handler function that will be called
        // at some later time when the HTTP server's response arrives. This kind of
        // asynchronous programming is very common in client-side JavaScript.
        req.onreadystatechange = function () {
            if (req.readyState == 4 && req.status == 200) {
                // If we get here, we got a complete valid HTTP response
                var response = req.responseText;     // HTTP response as a string
                var lenders = JSON.parse(response);  // Parse it to a JS array

                // Convert the array of lender objects to a string of HTML
                var list = "";
                for (var i = 0; i < lenders.length; i++) {
                    list += "<li><a href='" + lenders[i].url + "'>" +
                            lenders[i].name + "</a>";
                }

                // Display the HTML in the element from above.
                ad.innerHTML = "<ul>" + list + "</ul>";
            }
        }
    }

    // Chart monthly loan balance, interest and equity in an HTML <canvas> element.
    // If called with no arguments then just erase any previously drawn chart.
    function chart(principal, interest, monthly, payments) {
        var graph = document.getElementById("graph"); // Get the <canvas> tag
        graph.width = graph.width;  // Magic to clear and reset the canvas element

        // If we're called with no arguments, or if this browser does not support
        // graphics in a <canvas> element, then just return now.
        if (arguments.length == 0 || !graph.getContext) return;

        // Get the "context" object for the <canvas> that defines the drawing API
        var g = graph.getContext("2d"); // All drawing is done with this object
        var width = graph.width, height = graph.height; // Get canvas size

        // These functions convert payment numbers and dollar amounts to pixels
        function paymentToX(n) {
            return n * width / payments;
        }

        function amountToY(a) {
            return height - (a * height / (monthly * payments * 1.05));
        }

        // Payments are a straight line from (0,0) to (payments, monthly*payments)
        g.moveTo(paymentToX(0), amountToY(0));         // Start at lower left
        g.lineTo(paymentToX(payments),                 // Draw to upper right
                amountToY(monthly * payments));
        g.lineTo(paymentToX(payments), amountToY(0));  // Down to lower right
        g.closePath();                                 // And back to start
        g.fillStyle = "#f88";                          // Light red
        g.fill();                                      // Fill the triangle
        g.font = "bold 12px sans-serif";               // Define a font
        g.fillText("Total Interest Payments", 20, 20);  // Draw text in legend

        // Cumulative equity is non-linear and trickier to chart
        var equity = 0;
        g.beginPath();                                 // Begin a new shape
        g.moveTo(paymentToX(0), amountToY(0));         // starting at lower-left
        for (var p = 1; p <= payments; p++) {
            // For each payment, figure out how much is interest
            var thisMonthsInterest = (principal - equity) * interest;
            equity += (monthly - thisMonthsInterest);  // The rest goes to equity
            g.lineTo(paymentToX(p), amountToY(equity)); // Line to this point
        }
        g.lineTo(paymentToX(payments), amountToY(0));  // Line back to X axis
        g.closePath();                                 // And back to start point
        g.fillStyle = "green";                         // Now use green paint
        g.fill();                                      // And fill area under curve
        g.fillText("Total Equity", 20, 35);             // Label it in green

        // Loop again, as above, but chart loan balance as a thick black line
        var bal = principal;
        g.beginPath();
        g.moveTo(paymentToX(0), amountToY(bal));
        for (var p = 1; p <= payments; p++) {
            var thisMonthsInterest = bal * interest;
            bal -= (monthly - thisMonthsInterest);     // The rest goes to equity
            g.lineTo(paymentToX(p), amountToY(bal));    // Draw line to this point
        }
        g.lineWidth = 3;                               // Use a thick line
        g.stroke();                                    // Draw the balance curve
        g.fillStyle = "black";                         // Switch to black text
        g.fillText("Loan Balance", 20, 50);             // Legend entry

        // Now make yearly tick marks and year numbers on X axis
        g.textAlign = "center";                          // Center text over ticks
        var y = amountToY(0);                          // Y coordinate of X axis
        for (var year = 1; year * 12 <= payments; year++) { // For each year
            var x = paymentToX(year * 12);               // Compute tick position
            g.fillRect(x - 0.5, y - 3, 1, 3);                 // Draw the tick
            if (year == 1) g.fillText("Year", x, y - 5); // Label the axis
            if (year % 5 == 0 && year * 12 !== payments) // Number every 5 years
                g.fillText(String(year), x, y - 5);
        }

        // Mark payment amounts along the right edge
        g.textAlign = "right";                         // Right-justify text
        g.textBaseline = "middle";                     // Center it vertically
        var ticks = [monthly * payments, principal];     // The two points we'll mark
        var rightEdge = paymentToX(payments);          // X coordinate of Y axis
        for (var i = 0; i < ticks.length; i++) {        // For each of the 2 points
            var y = amountToY(ticks[i]);               // Compute Y position of tick
            g.fillRect(rightEdge - 3, y - 0.5, 3, 1);       // Draw the tick mark
            g.fillText(String(ticks[i].toFixed(0)),    // And label it.
                    rightEdge - 5, y);
        }
    }
</script>
</body>
</html>
